How dinosaur footprints form
How did prints that dinosaurs left in mud more than a hundred million years ago turn to stone, and what can footprints tell us that bones cannot?
From mud to rock
Most dinosaur footprints formed on ground that was damp but not too soft, such as riverbanks, lake shores or beaches. If the mud was too runny, the print collapsed; if it was too dry and hard, the foot left nothing behind. Sharp prints only form when conditions are "just right".[1][3]
After a dinosaur walked by, the print had to dry in the sun until its surface was firm, then be gently covered by a new layer of sediment, such as sand or mud washed in by water, without wiping it out. Layer after layer then built up on top, and their weight and the minerals seeping through slowly turned the sediment into rock over millions of years.[1][3]
When the Earth's crust was later uplifted and erosion stripped away the rock above, the prints hidden between the layers came back into view.[3]
True tracks, undertracks and casts
Not every print on a rock surface is the original track. Sometimes it is an "undertrack", where the pressure of the foot was passed down into the layer below. Undertracks are usually larger and blurrier than true tracks.[1]
Sometimes the sediment that covered a print filled it completely, forming a "natural cast" that stands up from the rock surface like an upside-down mold of the dinosaur's foot.[1]
What footprints tell us
Footprints are "trace fossils" that record the behavior of living animals, unlike bones, which only show an animal's shape after it died.[2]
From the length of a footprint and the distance between steps, scientists can estimate hip height and walking speed using formulas developed from living animals. Several parallel trackways suggest that some dinosaurs may have walked in groups, and the near absence of tail-drag marks confirms that dinosaurs held their tails off the ground.[1]
The catch is that we usually cannot tell exactly which dinosaur genus made a track. Scientists therefore name footprints separately from dinosaurs; these names are called "ichnotaxa".[4]
Dinosaur footprints in Thailand
The sandstones of the Khorat Group in northeastern Thailand hold many footprint sites, with three-toed tracks of meat-eating dinosaurs as well as tracks of plant-eaters. Several are open to visitors in national parks and wildlife sanctuaries, such as Phu Luang in Loei province.[5]
If you visit, don't step on the prints or pour water on them for photos. Sandstone weathers easily, and tracks that have lasted more than a hundred million years can be damaged within a few years.
Sources and further reading
More articles
- Why we say "dinosaur fossils", not "dinosaur bones"
- How do we know how old a dinosaur is?
- The first dinosaur fossils (in the world and in Thailand)
- How dinosaurs get their names
- How many Thai dinosaurs are there, and where were they found?
- Are birds really dinosaurs?
- The day the dinosaurs disappeared
- Animals people think are dinosaurs, but aren't
- Jurassic Park: how much of it is true?
- How do we know what colour dinosaurs were?
- Dinosaur eggs and babies
- How did long-necked dinosaurs get so big?